The present invention relates to signal pickup devices in apparatuses for reproducing recorded signals from rotary recording mediums. More particularly, the invention relates to a signal pickup device capable of tracking control, in an apparatus for reproducing a video signal from a rotary disc on which the video signal has been recorded on a spiral track or concentric circular tracks, in a manner such that a reproducing stylus for reproducing the video signal traces accurately on the track, and capable of jitter compensation in a manner such that a reproducing stylus moves in the longitudinal direction of track.
Heretofore, there have been apparatuses of the type wherein, for example, a rotary disc (referred to as "disc" hereinafter) on which a video signal has been recorded on a spiral track as variations in the geometrical shapes corresponding to an information content is rotated, and a reproducing transducer, that is, a signal pickup device is caused to trace relatively the spiral track thereby to reproduce the recorded video signal. In a signal pickup device of this character, it is necessary that the signal pickup device trace the track accurately. For this reason, it is necessary to provide means for detecting any tracking deviation of the signal pickup device relative to the above mentioned track on the disc and controlling, in response to this error, the position of the signal pickup device so that it will trace accurately over the track thereby to accomplish tracking control.
The present applicant has previously proposed a signal pickup device in which a permanent magnet member of the shape of a rectangular parallelepiped and having magnetic poles on the opposite lateral faces thereof is fixed to the proximal end of a cantilever. A first coil is disposed to surround the permanent magnet member. Second coils are disposed to confront with each magnetic pole of the permanent magnet member. In this device, when the tracking control signal current and the jitter compensation signal current respectively flow through the first coil and the second coils, the permanent magnet member is energized to rotate about a hypothetic vertical axis thereof and to displace in the longitudinal direction thereof, according to Fleming's left-hand rule, whereby the cantilever is rotated and displaced in the axial direction of the cantilever. Tracking control operation and jitter compensation operation are thereby carried out.
This signal pickup device, however, is accompanied by following problems, due to its construction. The permanent magnet member, which is a moving member, is required to be miniaturized and light-weighted, which causes inevitably the magnetic force thereof to be small. Accordingly, the driving force of the permanent magnet member becomes small, whereby the operation of the cantilever with large driving force becomes difficult, thereby arising difficulties that the prescribed control operation cannot be performed.